Conventionally, it is often the case that a converter includes a diode clamp-type 3-level circuit. With respect to such a converter, there has been a demand for miniaturization of a converter device using a low loss device such as a silicon carbide element. However, so far as currently available silicon carbide elements are concerned, there is no known element which withstands a high voltage. Accordingly, it is necessary to arrange the silicon carbide elements in series or to adopt multi-leveling where the silicon carbide element is combined with an existing silicon element. Of these two methods, the arrangement of the elements in series has drawbacks such as an increase in resistance losses, an increase in the total number of components and the necessity for balance control. Accordingly, the multi-leveling is viewed as more practical than the arrangement of the elements in series.
With respect to the multi-leveling technique, a flying capacitor method has been proposed as a method where the number of switching elements can be decreased against the number of levels of an output voltage. However, the number of capacitors is increased in such a method.
A diode clamping method has been proposed for limiting the increase in the number of capacitors in a multi-leveling device. In the diode clamping method, a balance circuit having a filter capacitor voltage is necessary so that there is a possibility that the size of the converter device is increased. Besides the diode clamping method, other methods include a cascade method where AC (alternating current) input/output points of single-phase full-bridge converters (inverter) are connected in series and a gray scale control method.
In the related art, the number of capacitors can be decreased while ensuring a high electrical breakdown strength. However, the number of switching elements is increased with the number of levels of the output voltage and hence, the miniaturization of a converter device is difficult.
Accordingly, the use of a multi-level converter where a plurality of different level converters are connected in series is considered. However, a circuit which suppresses an overvoltage of a capacitor of the level converter becomes necessary for each level converter. Although a protective circuit for suppressing the overvoltage includes a discharge resistor for performing a rapid discharge, the protective circuit is required to have a capacity which can withstand a large electric current so there is a tendency that the protective circuit becomes larger. Accordingly, it is difficult to reduce the size of the power conversion apparatus.